When you make periodic snapshots of a volume, the snapshots are
incremental, and only the blocks on the device that have changed since
your last snapshot are saved in the new snapshot. When you delete a
snapshot, only the data not needed for any other snapshot is removed.
So regardless of which prior snapshots have been deleted, all active
snapshots will have access to all the information needed to restore
the volume.

You cannot delete a snapshot of the root device of an EBS volume used
by a registered AMI. You must first de-register the AMI before you can
delete the snapshot.

For more information, see
Deleting an Amazon EBS Snapshot
in the Amazon Elastic Compute Cloud User Guide .